Proper use of an amp clamp can find your circuits- especially if you can cycle the load.
That's an idea which occurred to me for a helpful gizmo -- Something like Scott's drill idea, but automated for one-man tracing.
Build a plug-in unit which will automatically cycle a load on and off. The load could be a few 100W bulbs, or if the duty cycle is kept down a dummy-load resistance in the box.
If you wanted to get fancy, you could make both the load and the timing adjustable for versatility. Once it's plugged in, you could then use your amp-clamp at the panel to look for the "signature" of your device (e.g. 1 sec. on at 10 amps, 3 secs. off, or whatever you've chosen).
